The present invention relates to optical bar code scanners, and more specifically to an apparatus and method for equalizing the signal strengths of different scan lines.
Optical scanners are well known for their usefulness in retail checkout and inventory control. Optical scanners generally employ a single laser source, the light from which is collimated and focused to produce a scanning beam. They may additionally employ a mirrored spinner to direct the beam against a plurality of stationary mirrors, and a detector to collect the beam after it is reflected by a bar code label. The pattern produced by such a scanner is characterized by lines oriented at various angles to one another.
A problem with the typical optical scanner is that scan lines from different pattern mirrors have different field strengths and produce returning light beams which have different signal strengths at the photodetector. This difference in field strength is caused by many factors, including differences in the incidence angles at the pattern mirrors and at the aperture glass. Collected laser power may vary by as much as 30%.
Mirrors used in optical scanners are evaporated coated first surface mirrors. The factors which determine the amount of reflectance are substrate type and flatness, and coating material and thickness. Typical scanners employ mirrors coated with aluminum or gold, which produce the highest reflectance for the light wavelengths used in these scanners.
Prior scanner designs are not equipped to process signals having strengths that are outside a predetermined range of signal strengths. The gain of signal processing circuitry must be set to amplify the weakest signals, but not to the point that stronger signals cause saturation. Signals that are too weak are not decoded. Signals that are too strong adversely affect the operation processing circuitry within the scanner.
Therefore, it would be desirable to provide an apparatus and method for equalizing the signal strengths of different scan lines to ensure that returning light from all of the scan lines is processed.